C150 flight Instructor reported engine malfunction inflight. Conducted precautionary off airport landing.

Narrative

During a routine training flight we experienced a sudden loss of engine power along with an extremely rough vibration. As Instructor; I immediately transferred control of the aircraft to me; engaged carburetor heat; verified mixture was full rich; verified fuel shutoff handle was open and mags were selected to Both. Further assessing the situation; the power continued to decline to approximately 1200 -1400 RPM at any throttle setting; and the vibration continued to be excessive especially when I engaged higher throttle settings. Rather than risk a total engine failure while attempting to fly back to an airport; I decided to execute a precautionary landing. A suitable landing area was close; a field that was relatively flat with no obstructions. I established my landing approach and executed a soft field landing without incident. After landing; the Fire Chief of ZZZ and an EMS driver arrived. I let them know everything and everyone was OK and asked him to contact the FAA. I contacted my flight school owners and they dispatched to the aircraft along with their Chief of Maintenance.The aircraft will be trailered back to the hangar for further inspection and repair of the engine.The aircraft is a Cessna 150. All required aircraft inspections were up to date as of this flight. We departed ZZZ with 16 gallons of fuel on board; and just under 6 quarts of oil was indicated when checked. A complete pre-flight inspection and weight and balance was conducted prior to flight including the proper sumping of the fuel system.